Salted hides and skins are the major raw materials for the tanning industry. In leather making process, the first operation carried out is removal of the salt in the hides and skins through repeated washings. The hair and flesh are loosened by treatment with lime and sodium sulfide and then removed using knives. The hides and skins are then agitated with acid salts like ammonium chloride for removal of residual lime. The operation is called deliming, which is usually carried out in tanning drum. The resulting delimed hide and skins are subjected to pickling using 10% salt (all percentages mentioned are on the basis of weight of hides and skins) and 1% mineral acid in the hitherto known process, basically to condition the material prior to chrome tanning which is usually carried out by treatment with basic chromium sulfate (BCS). In other words, pH of the delimed stock, which is around 8.0 is brought down to 2.8 to 3.0 during pickling to facilitate uptake as well as penetration of chrome in the cross section of the stock to be tanned. While the penetration of BCS in hides and skins during chrome tanning is carried out at a pH 2.8 to 3.0 the same is raised to 3.8 to 4.0 for fixation, which is known as basification in the industry. Alkaline salts of weak organic acids which are normally referred to as masking salts are added during chrome tanning process to prevent surface fixation of chromium in hides and skins.
The major limitation associated with the conventional chrome tanning process is the generation of effluents with significant quantities of total dissolved solids and chromium. In fact pickling and chrome tanning contribute significantly for total dissolved solids in the effluent during chrome tanning. Presence of large quantities of total dissolved solids in effluent makes the treatment of tannery effluents a cost ineffective proposal. Till now, no viable and cost effective solution is available for secured treatment of total dissolved solids. Minimising or eliminating the problem of total dissolved solids has been a topic of research interest for quite sometime. U.S. Pat. No. 4,484,925 deals with the treatment of leathers using non-soluble basic metallic salts with or without conventional mineral tanning salts in order to impart mycostatic, antimycotic and anti fungal properties to the leathers. Whereas, the present invention deals with elimination of a process step, pickling in the conventional mineral tanning, without any change in characteristics of the final leathers
Another U.S. Pat. No. 4,622,156 deals with the treatment of pickled skins and hides with chromium tanning agent using alumina hydrate as chromium fixing agent by different approaches, while the present invention deals with elimination of pickling itself and execution of tanning at deliming stage itself.
One more U.S. Pat. No. 5,505,708 relates to a process for the preparation of tanned hides using metals other than chromium and to compound to such process. The patent deals with non-chrome tanning method. However, it mentions that metallic salt can be chosen from chromium, aluminium, titanium and zirconium. In addition, use of metallic salts other than aluminium is not substantiated through examples. In the present invention, the methodology adapted was to control the hydrolysis of chromium using salts of alkaline metals. The present invention has overcome the limitations of hitherto known pickle free systems and approach is different. The present invention claims a use of conventional chrome tanning salt and alkali metal salts eliminating a polluting step in process to produce chrome tanned leathers.
Gupta et al. (Journal of Indian Leather Technologists Association, 41, 24–26, 1992) developed a pickle free alum—chrome combination tanning system using only 2% of salt thereby reducing use of salt to an extent of 8%. However, use of aluminium in the system impairs the feel and softness of the leathers. Further, the addition of salt is not eliminated completely but is reduced. A low salt pickling system has been reported by Sridharnath et al (Leather Research and Industry Get-together Poster Session, CLRI 2001) which again pertains to use of 2% salt based pickling. The low salt pickling system has been found to be suitable for skins predominantly and again does not eliminate use of salt completely. The strength characteristics of the leathers were found to be adversely affected when salt was eliminated completely. An improved product-process for cleaner chrome tanning in leather processing has been developed by Suresh et al (Journal of Cleaner Production, 9, 483–491, 2001) where in the conventional pickling is dispensed with and use of a synthetic chrome tanning agent instead of BCS is resorted to. The pH of the process was maintained between 4.0The major limitation of the system is that it is applicable for skins only and the system calls for post tanning modification in processes.
The present invention relates to a modified process of subjecting the delimed hides or skins to chrome tanning under certain conditions, as disclosed herein below, to facilitate avoiding conventional salt, thereby overcoming the problem of total dissolved solids (TDS), which has lately been a burning issue in the leather industry.
The definitions of certain terms used in the present specifications, which are normally used in the leather industry, are given below.
    Bating: The process prior to tanning proper where the fibres of a hide or skin which have been plumped or swollen by liming are reduced and softened, thus assuring pliability in the product. The word is a form of “abate” in the sense of reduce.    Beamhouse: The area of the tannery where the unhairing and liming processes are carried out. Before modern machinery the processes of fleshing, scudding and others were carried out over specially designed wooden beams using curved two handled knives with sharp or blunted edge according to the operation. Hence the name.    Deliming: A process preparatory to tanning proper to reduce the swollen and rigid fibres of a hide or skin after liming.    Drums: The collective term for the rotary vessel in which many processes of tanning are now carried out.    Side Leather: Half a hide cut along the backbone. Large hides are cut in half down the backbone to make them easier to handle in the tannery. Each piece is called a side. Most shoe leather made from cattle hides is produced as side leather, while most upholstery leather produced from hides is processed as whole hides. Increasingly the cutting of hides into sides is done after tanning, at the wet blue stage.    Pickling: A preliminary process for preparing hides and skins for tanning, largely by adjusting the pH with acid and controlling the swelling with salt. It is also use as stable way of holding material, after unhairing, for transport between plants and countries and for trading.    Liming: A process preliminary to tanning, which serves one or both of two purposes: (1) to loosen the hair or wool on a hide or skin, preparatory to dehairing; and (2) to plump or swell the fibres as of the processes necessary to prepare the substance for the action of the tan liquors. Liming is always required for the latter of these purposes.